Megavoltage X-ray Dose Enhancement with Gold Nanoparticles in Tumor Bearing Mice.

One of the applications of gold nanoparticles (GNPs) in medicine is radiation dose-enhancing effect. Although there are many simulations, in vitro and in vivo evidence that GNPs can enhance significantly the radiation dose effect of orthovoltage beams. These beams compared with megavoltage (MV) beams, have limited applications in radiotherapy. In order to evaluate GNPs radiosensitization performance with MV beams in-vivo, we used two most clinically used X-ray beams (6 and 18 MV) with the dose of 20 Gy for each mouse. Intratumoral injection of 50 nm GNPs with the concentration of 5 mg ml(-1) was applied to melanoma tumor growing in the left leg of 7 to 8 mice in 4 control and treatment groups of C57BL/6 mice. Albeit, using 10 cm plexiglass jig phantom in the beam path improved the radiation - treatments, the statistical differences between groups were not significant. According to the results, it is concluded that mice can be treated with smaller tumors and higher concentrations of GNPs in MV radiation beams.

adiotherapy is an important method in cancer treatment, but it can be held responsible for biological damages as radiations can also induce a decrease in normal tissues. Improvements in this cancer therapeutic method have begun in the past decades by performing intensity modulated radiation therapy (IMRT), multileaf collimators (MLCs) and stereotactic radiosurgery. Radiosen-sitization of tumors with safety media is an alternative method to improve the discrimination between tumors and normal tissues (1)(2)(3). Initially, this technique was known as X-ray phototherapy with iodine and gadolinium contrast media (4)(5).
Then it was introduced as contrast-enhanced radiation therapy or CERT (6-7) but in numerous papers, this method was mentioned as gold R Megavoltage X-ray Dose Enhancement with GNPs 119 Int J Mol Cell Med Summer 2013; Vol 2 No 3 nanoparticle radiation therapy or GNRT (8)(9). In recent years, this method has been of an increasingly interest by the use of gold nanoparticles radiosensitize contrast media due to its high atomic number and bio-compatibility (8). Through the progress in nanotechnology, the synthesis of various gold nanostructures such as spherical gold nanoparticles (GNPs), gold nanorods (GNRs), gold nanoshells (GNSs) and gold nanocages (GNCs) in cancer therapeutics have been made (2). Gold nanostructures can be applied as radiation sensitizers, anticancer drug enhancers, heat generators and also effective drug carriers.
The first step of this study was a series of computer simulations (10). The Monte Carlo method was described in several studies in the last years. These studies have all shown the radiosensitization of GNPs, especially with kilovoltage radiation exposures (11)(12)(13)(14). But recent studies have also shown the radiosensitization effect at megavoltage beams (15)(16)(17). This effect can be understood on the basis of Local Effect Animal studies have also shown that gold microspheres or nanoparticles can enhance radiobiological effect of radiation dose (18,(26)(27)(28)(29)

Cells and mice
One T 25    Digital radiographic images of the mice were used to identify intratumoral distribution of GNPs.

Characteristics of irradiation
The mice were anesthetized with Xiluzin and MV X-ray beams. The lower slab was made from wax with eight places for eight tumor bearing legs of mice (Fig. 2). Radiation was applied to an 8.5 cm × 20 cm field size at 108 cm source-to-surface distance (SSD). The total 20 Gy dose was delivered with five 576 MU's separated very short intervals.
The time within each fraction was at least 5 minute.

B16F10 melanoma cells were cultured and grown
subcutaneously in the left legs of the C57BL/6 mice. The growth of the tumor and mice survival  were also considered indices for the different treatments groups. Figure 1   Kaplan-Meier survival graphs (Fig. 4) have also shown the differences between groups. The GNPs without radiation could not stop the growth of the tumor and increased the survival of the mice.
All mice receiving either no radiation or GNPs without radiation died during the span of one month.

Discussion
In this study, in vivo GNPs radiosensitization was evaluated in radiotherapy with clinical megavoltage X-ray beams. We used the same geometrical set-ups utilized in the separate simulation and dosimetric study (10). DEF between 8-10% was obtained in deep tumor-like insert phantom (10 cm depth) that contained GNPs and irradiated with 18 MV.
It appears that the size of tumor is a very important parameter in radiotherapy. This study is one of the novel works that deals with high energy X-ray beams. It is revealed that tumor size and its response to the various treatments determine the shape of cell survival probability. We carried out treatments with about 500 mm 3 tumor size while Hainfeld and Chang performed their in vivo studies with very small tumor sizes (100 mm 3 approximately) (26)(27). Another motive was GNPs desired concentration in the tumor. Thus, tumor size was only controlled in the first month after irradiation with 18 MV (Fig. 1). This effect occurred much less